As used herein the terms cathode ray tube (CRT), picture tube, and kine have the same meaning and are used interchangeably.
Modern television receivers differ from their predecessors in that the discrete circuitry of the older models is being replaced with integrated circuitry capable of performing ever more and complex functions. Some advantages inherent in employing integrated circuits include lower manufacturing costs due to few parts to stock and assemble, greater reliability, less circuit board space needed, and fewer solder connections. One integrated circuit (IC) is the Luma/Chroma IC used by RCA in the CTC 140 chassis.
As the number of functions performed by an IC increases, the circuitry on the IC becomes more densely packed with the result that the power dissipation undesirably increases. One way to limit the power dissipation of an IC is to limit its output voltage swing to lower values than was necessary for less densely packed ICs. For example, a relatively complex IC may have an output voltage swing of 2.2 volts peak-to-peak as compared to less complex integrated circuits which may have typical output voltage swings of 3.3 volts peak-to-peak.
In order to compensate for a reduction of signal level, it may be necessary for the kine driver circuitry to be operated at higher gain levels to produce a satisfactory picture. For example, it may be necessary to increase the gain of a kine driver circuit from approximately 40 to approximately 60.
It is herein recognized that unfortunately, operating the kine driver circuits at a higher gain may cause a new problem that the Applicant calls focus flutter.
Focus flutter occurs shortly after a television receiver is turned on as follows. When power is first applied to a television receiver the CRT filament is cold and there is no emission from the cathodes. There is therefore no beam current, causing the tube cathodes to appear as open circuits. Since there is no beam current, the beam current limiter does not reduce contrast or brightness. Therefore, since at maximum contrast control setting the video output signal from the IC is amplified to the greatest extent, and there is no beam current flowing, the high gain kine drivers will saturate.
In normal operation, the cathode potential is about 150-160 volts higher than the grid (Gl) potential and beam current is, as a result, held within a normal range. However, the above-described condition of kine driver saturation causes the cathode voltage to fall to a level at or below the level of the voltage on the grid (Gl) to produce a so called "zero bias" condition. In this "zero bias" condition, when the CRT filament has heated the cathodes enough to have emission, excessive beam current will begin to flow.
With some picture tubes, excessive beam current can be partially intercepted by the focus electrode, causing focus current to flow. This, in turn, causes the focus voltage to drop, and defocuses the beam, causing even more beam current to strike the focus electrode. This positive feedback phenomenon manifests itself as a brightness fluctuation of the picture.
The focus flutter problem occurs typically for 1 to 2 seconds after approximately an 8 second warm-up delay, during a turn-on with a white field when the brightness and/or contrast controls are set to their maximum positions. The problem is relieved after 1 to 2 seconds because, as the beam current begins to flow, the kine drivers come out of saturation and the zero bias condition disappears.
The problem of focus flutter is a function of the geometry of the electron gun assembly of the CRT. Specifically, it has been found that the problem probably will not be exhibited with 20 inch (diagonal measure) picture tubes, such as the RCA A51ACG14X, while the problem is likely to be exhibited with 26 inch (diagonal measure) picture tubes, such as the RCA A66ADT14X. Therefore, it would seem that the problem could be alleviated by redesigning the electron gun assembly of the picture tube. That solution, however, is undesirable because the same type of picture tube is typically used with many different chasses, some of which may not use high gain kine drivers and therefore not have the problem. Moreover, there is a the high cost associated with producing new picture tubes.